By parallel implementation, digital signal processing semiconductor devices have extended the operating speed to tens of gigabit per second. With this development, a digital coherent optical communication that enables multilevel transmission using the digital signal processing is studied as a long haul and large capacity communication technique and is making progress.
In the digital coherent optical communication, both intensity and phase of an optical signal is manipulated with a digital circuit. As a result, the chromatic dispersion and the fast-fluctuating polarization mode dispersion (PMD) in an optical fiber are easily compensated, and high sensitivity due to the coherent receiving is also expected.
A receiver of multilevel coherent optical transport system with the digital processing disclosed by “Digital Coherent Receiver Technology for 100-Gbps Optical Transport Systems” (Rasumussen, Hoshida, Nakajima, FUJITSU. 60, 5, P. 476-483, September, 2009) is made up of, in most part, a digital circuit that estimates a phase variation of an incoming lightwave and a local oscillator lightwave of the receiver after the superimposed incoming lightwave and local oscillator lightwave is converted into an electrical signal and into a digital signal.
However, very low phase-noise lasers required for optical transmitters and local oscillators of an optical receivers in multilevel coherent optical communication such as 16 quadrature amplitude modulation (QAM) and 64QAM that is expected to be in practical use in the future. This demand calls for a laser having a narrower spectral linewidth by the aid of, for example, the quadrature phase shift keying (QPSK). In order to fulfill this requirement only by laser property improvement, the laser cavity length becomes longer and the laser becomes larger and more expensive. Thus the realization by an actual communication device is considered to be difficult.
The digital coherent receiver compensates the chromatic dispersion for a digital signal. Incoming light undergoes chromatic dispersion in an optical fiber and dispersion compensation at a digital circuit of a receiver. Thus the degradation due to the chromatic dispersion tends to zero. On the other hand, the local oscillator lightwave from the receiver undergoes only the dispersion compensation at the digital circuit of the receiver since the local oscillator lightwave does not travel through the optical fiber. Thus, when the local oscillator lightwave includes significant phase noise, the degradation of waveform emerges in proportion to the breadth of wavelength of the local oscillator lightwave.
“Phase Noise Cancellation in Coherent Optical Receivers by Digital Coherence Enhancement” (M. Secondini et al., ECOC, September, 2010, P 4. 17) discloses a digital coherent receiver that measures the phase variation of a local oscillator lightwave that has not been superimposed on a signal lightwave, and based on the measurement, conducts phase rotation processing at a digital circuit. Other related documents are, for example, “Coherent optical OFDM: has its time come? [Invited]” (William Shieh, Xingwen Yi, Yiran Ma, and Qi Yang, Journal of Optical Networking, Vol. 7, Issue 3, pp. 234-255, 2008) and “Local oscillator phase noise induced penalties in optical coherent detection systems using electronic chromatic dispersion compensation”, (C. Xie, OFC2009 OMT4, 2009).
However, the above-noted documents possess a problem of large power consumption of the phase rotation processing. Nyquist theorem indicates that in order to reconstruct data from an incoming lightwave that has underwent the chromatic dispersion on the path and has large variation within one symbol, it is necessary to process signals twice as fast as the symbol rate. For a disturbed wave form due to the chromatic dispersion, the phase rotation processing is performed at a higher rate and thus more parallel circuits need be combined, resulting in more power consumption in the digital signal processing circuit.
Furthermore, compensation of the phase variation (frequency variation) exceeding 2π according to the document of Secondini et al. requires a manipulation of a phase term in the trigonometric function instead of a phase rotation matrix operation. Even the signal processing for a non-polarization-multiplexed signal becomes complicated. In addition, when a polarization multiplexed optical signal is transmitted, the phase variation (frequency variation) beyond 2π can be compensated, in principle, only after two polarized signal are separated, which has not been mentioned in the document of Secondini et al.